1. Field of the Invention
The invention relates to the filed of charging systems for batteries and, in particular, to a charging system that prevents over charging of individual cells of a battery composed of a plurality of cells in series.
2. Description of Related Art
Certain types of batteries, such as Lithium-ion batteries are subject to damage if over charged. If there is a single battery cell in use, charging presents no problem. A simple charging system can be used that senses when the cell is fully charged and thereafter trickle charging are used to maintain the charge. One type of system switches off the charging current; for example U.S. Pat. No. 5,539,299 "Protection Switch For A Battery Powered Device" by J. M. Ferandez, U.S. Pat. No. 5,853,908 "Protection Device For Secondary Batteries" by T. Okutoh and U.S. Pat. No. 5,892,145 "Battery Pack Unit, Control Method Therefore, Electronic Equipment Driven Power Supplied From Same Battery Pack Unit, And Recording Medium On Which Program For Controlling The Same Battery Pack Unit Is Recorded" by Y. Eguchi.
U.S. Pat. No. 5,049,804 "Universal Battery Charging System And A Method" by P. D. Hutchings and U.S. Pat. No. 5,164,652 "Method And Apparatus For Determining Battery Type And Modifying Operating Characteristics" by R. M. Johnson, et al., and U.S. Pat. No. 5,420,493 "Power Supply And Battery Charger" by A. Hargadon, et al. all disclose charging systems that can be used to charge various types of batteries. There are literally hundreds of battery charger designs for charging a single battery cell. However, when a battery is composed of a plurality of cells in series, such systems will not prevent an individual cell from being over charged, especially when the battery is composed of a plurality of Lithium-ion cells.
U.S. Pat. No. 5,969,502 "Battery Charger Capable Of Independently Charging Electromagnetic Cells". P. Beard discloses a charging system for a pair of cells arranged in series wherein the system independently controls charging of each cell. A resistor limits the current to the battery and a voltage equalizer is used to balance the cell voltage between the cells during charging. However, while this will work for a pair of cells in series it is not useful for a higher number of cells.
In U.S. patent application Ser. No. 09/493,581 "Battery Cell By-Pass Circuit" by S. W. Mumaw, et al. filed Jan. 28, 2000 (common assignee), an approach is disclosed wherein a system for limiting the charging voltage applied to an individual cell of a series of cells forming a battery, the cells being charged in series It includes a voltage detection circuit to sense the actual voltage level of the individual cell. A voltage comparison circuit is used to compare the actual voltage level of the individual cell with a reference value and to provide an error signal representative thereof. A third circuit, designed to be responsive to the error signal, is provided for bypassing charging current from the individual cell if the actual voltage level is above the specific voltage level and converts the excess charging current to heat. The circuitry also allows charging of the individual cell if the actual voltage level is equal or less than the specific voltage level. The disadvantage of this circuit is that no attempt is made to reclaim the energy by-passed around the individual cell.
Thus, it is a primary object of the invention to provide a circuit for preventing the overcharging of an individual cell of a battery composed of a plurality of cells in series.
It is another primary object of the invention to provide a circuit for preventing the overcharging of an individual cell of a battery composed of a plurality of cells in series wherein each individual cell is provided with a charging by-pass circuit.
It is a further object of the invention to provide a circuit, which allows current to by-pass all cells at a specific state of charge with the purpose of equalizing the state of charge of each individual cell with the battery.
It is a further object of the invention to provide a circuit for preventing the overcharging of an individual cell of a battery composed of a plurality of cells in series wherein each individual cell is provided with a charging by-pass circuit and where the by-pass circuit reclaims the energy by-passed around the cell or cells.